I think after this year they will go with just the iPhone moniker especially since this what would be a 7S year will produce a new design ground up based on what the last year and current rumors are. It only makes sense just iPhone and iPhone plus they are going to leave the numbers behind for this anniversary model.
Probably going to be the iphone 8 since it will be the 10 year iphone anniversary. I am already predicting that the phone will recieve a redesign with trimmed bezel, wireless charging, slightly improved screen, and some upgrades to the camera and processor. I am sure the next iphone will will be cool and all but after paying full for this iphone 7 plus, I won't be upgrading again for at least two or three years.
Or will Apple go straight to the 8?
Maybe Apple will pull a Samsung and Skip a number or two...or three. iPhone 10? There is precedence as Apple didn't have an iPhone 2 or iPhone 3 ... went straight to iPhone 3g.